93 caprice classic wagon

When driving to work about every other day, after about 20 minutes on the road my check engine light comes on and my car begins to act as if it is going to stall,seems to get better if I put car in neutral but light stays on and car returns to sputter when put back in drive, after about 5 minutes of this the engine will die when the car comes to its next stop or if I remove my foot from the gas. The car will restart after I turn key completly off and then back on, car will run fine for a while then do the same thing, its almost like it only happens when engine gets to a certain temp. I have had a tuneup, replaced fuel filter, flushed radiator changed thermestate and complete oil change, just got the car back yesterday and it died on me again this morning, I am thinking it could be the o2 sensor. Also, my speed ometor does not always work, sometimes it falls to zero and doesnt go back up, other times it works fine, my mechanic said this had nothing to do with my check engine prob does it? Any advice or suggestions would be appreciated.

Thanks,
Mel

got any code's ???